Should we automatically flatten nested fields in bare rows queries?

The old Query Builder used to let people specify the `:fields` to be returned in a bare rows query which meant people could pick nested columns to show as top-level columns as they saw fit. Since you can't really do that in the current iteration maybe it makes sense to do it automatically rather than leaving them nested...

Or maybe not. Opened so we can discuss this idea. (Originally discussed in #1328)

Start by reviewing the original discussion in #1328 and locating the current bare rows query implementation. Clarify whether nested fields should be flattened automatically, including the expected behavior for nested columns. Done requires an agreed scope and decision, but the issue does not currently define an implementation or acceptance criteria.

Written by the indexing model from the issue text.